The present invention relates to electric welding machines, and relates more particularly to such an electric welding machine which uses a variable resistor to control the operation of an annular power transformer and an inductor so as to maintain the electric arc stiff during welding.
FIG. 1 shows a regular electric welding machine, which comprises a substantially rectangular transformer 10 and a movable iron core M3 at the center opening of the transformer 10 to close the magnetic line of force so as to limit electric current. The transformer 10 comprises an iron core, which consists of two iron core parts M1 and M2, a primary winding 20 and a secondary winding 30 respectively wound the iron core M1 and M2. The central iron core, namely, the movable iron core M3 is driven by a handle to move forward or backward in the transformer 10, and therefore the output current is regulated. The relationship between the position of the movable iron core M3 and the output current is shown in FIG. 2a, 2b, and 2c. When the movable iron core M3 is moved out of the transformer 10 as shown in FIG. 2a, the magnetic line of force from M1 is transferred completely to M2, and the electric welding machine receives the maximum value of electric current. When the movable iron core M3 is partially moved into the transformer 10 as shown in FIG. 2b, the magnetic line of force from M1 partially passes through M3, therefore less magnetic line of force is induced by M2, and less electric current is outputted. The value of output current is relatively reduced when the movable iron core M3 moves backwards inside the transformer 10. When the movable iron core M3 is completely moved back inside the transformer 10 as shown in FIG. 2c, the magnetic line of force from M1 is completely blocked up between M1 and M2, the output current reaches the minimum value. Because the movable iron core M3 blocks up a part of the magnetic line of force in between M1 and M2, the remainder of the magnetic line of force is transferred to M2. Because the induced magnetic line of force is constant, a constant value of electric current is given to the electric welding machine. However, this structure of electric welding machine is still not satisfactory in function. Because the primary winding 20 and the secondary winding 30 are respectively wound on the transformer 10 at two opposite sides, the course of the magnetic line of force is long. The size of the iron cores must be greatly increased. Therefore, this structure of electric welding machine is big and heavy (for example, an electric welding machine of 300A weights over 120 kgs). Because the passage of the magnetic line of force is long, the energy loss is high, and the efficiency is low. When not in welding, the loss of static current is quite high (the utilization percent is about 45%, the efficiency is about 45%, and the static current is about within 10-15A). FIG. 3 is a voltage-current curve obtained from tests made on the aforesaid structure of electric welding machine, showing the relationship between the end voltage and the end current, in which A indicates the arc current; B indicates the arc voltage. Furthermore, the transformer 10 is complicated and expensive to manufacture. The iron core M1 and M2 is made by: processing silicon steel sheets into substantially U-shaped elements, and I-shaped elements, then alternatively connecting the U-shaped elements and I-shaped elements into shape. This iron core production procedure takes much working time, and produces much waste material.